Numerous systems are known that sense vehicle steering corrections during a given time period as an indication of driver alertness. One such system is disclosed in U.S. Pat. No. 4,278,969 to Richard Woods, entitled "Driver Warning System". The '969 Woods device incorporates a light source and photocell mounted on the steering column that directs a light beam towards a strip having alternate bands of reflective and non-reflective material. During normal driving patterns, the steering wheel is corrected a given number of times during any predetermined time period. When steering corrections fall below the predetermined number, the driver is usually inattentive due to any of a number of reasons. An audible driver warning system during such conditions has been shown to be effective to arouse the driver to a state of alertness to prevent a vehicle accident. For example, the Woods system is coupled with vehicle speed sensing devices which make it inoperative below a certain vehicle speed so that when the vehicle is parked or moving at a relatively slow speed, the audible alarm will not be sounded even though the necessary steering corrections are not made within the given time period. One significant shortcoming of the Woods device is its reliance on optics for motion detection.
Optical devices used in a motor vehicle environment are subject to significant amounts of dirt, grime, grease and other likely contaminants. Such contaminants will likely interfere with and prevent the Woods device from functioning properly. A steering wheel movement detection device that is unaffected by such contaminants is needed.